The present invention relates in general to frequency discriminators and more particularly to a novel two-pole crystal monolithic frequency discriminator of improved operating characteristics.
Present day radio communications requirements, and particularly for voice communication, demand narrow-band operation so as to maximize a number of available channels. Conventionally, the deviation of a frequency modulation channel is restricted to five kHz or less. At high frequency operation, say, 150 mHz or above, the demodulation of such narrow-band carrier signals present significant difficulties, particularly since the more conventional demodulator apparatus or frequency discriminators are relatively complex in operation, not to mention the additional problems in tuning and alignment customarily required.
One approach to the foregoing has been the utilization of crystal discriminator arrangements which achieve very narrow passbands, without complex circuitry, and which demodulate the received carrier signals directly, or at least the heterodyned intermediate carrier signals of a relatively high frequency. The crystal discriminator referred to customarily comprises a plurality of electrodes, usually three pairs, on a single crystal blank or wafer. One such pair serves as the input resonator while the remaining two function as two output resonators. The input resonator is tuned to the carrier or intermediate frequency, as the occasion may be, with the two output resonators tuned to slightly higher and slightly lower frequencies, respectively. By making the various electrodes sufficiently massive to achieve energy trapping, significant decoupling occurs between the input and the output resonators, and a relatively narrow passband results. More particularly, the energy coupled out of the input resonator to each of the two output resonators forms respective stagger-tuned pass bands, which do not coincide with one another. The high and low band output frequencies may be suitably detected and combined subtractively to recover the desired modulation at audio frequencies as contained therein.
While the referenced prior crystal discriminators may be entirely satisfactory in certain applications, there nevertheless are others where they are not suited. Three-pole devices are obviously more costly and difficult to fabricate than the less complicated two-pole devices. For miniaturized electronic apparatus, such as, say portable, and hand held communication apparatus, space is an extremely important factor and may in fact be critical. Tuning, and the attendant circuit components required, simply cannot be tolerated. Yet, at the same time, there are instances where a somewhat wider bandwidth is desired, indeed is mandated, than what has heretofore been obtainable in prior two-pole monolithic crystal devices.